1. Field of the Disclosure
This disclosure generally relates to a sensing system and, more particularly, to an image sensor and an imaging system adopting analog buffers.
2. Description of the Related Art
A conventional image sensor generally has a plurality of sensing pixels arranged in array, wherein one operating method of the image sensor uses a rolling shutter, for example, to successively activate, with a control signal, each row of the sensing pixels in the image sensor to capture an image. Since each row of the sensing pixels is successively activated instead of simultaneously activated (i.e. start timings for each row of the sensing pixels to be exposed are different), the image captured by using the rolling shutter may cause distortion in capturing an image of a fast moving object.
Another operating method of the image sensor uses a global shutter, for example, simultaneously to activate all sensing pixels in the image sensor with a control signal so that each row of the sensing pixels starts to be exposed at an identical time to capture an image. Therefore, the image sensor using the global shutter may avoid the distortion problem.
However, in order to eliminate ambient light interference or reducing image noise when the image sensor using the global shutter is capturing an image, one solution is to directly perform a subtraction between two digital image frames through image post processing so as to obtain a differential image frame. For example, referring to FIGS. 1A and 1B, FIG. 1A is a timing diagram of a conventional image sensor 91 using the global shutter to capture an image. It is assumed that the image sensor 91 has 4 rows of sensing pixels R1-R4. In a first period P1, a light source is turned on for a predetermined time and the rows of sensing pixels R1-R4 are simultaneously exposed. Then, the light source is turned off and the rows of sensing pixels R1-R4 are successively read to output a first image signal. In a second period P2, the light source is turned off and the rows of sensing pixels R1-R4 are simultaneously exposed for the predetermined time, and the rows of sensing pixels R1-R4 are successively read to output a second image signal.
Then, referring to FIG. 1B, the first image signal outputted from the image sensor 91 is firstly converted into a first digital signal 9a through an analog to digital converter 93 and stored in a digital buffer 95. Then, the second image signal is converted into a second digital signal 9b through the analog to digital converter 93. Finally, the second digital signal 9b is subtracted from the first digital signal 9a to obtain a third digital signal 9c in which ambient light is eliminated. However, in this way, the digital buffer 95 has to be disposed in a system including the image sensor 91 and the image sensor 91 must successively output two image frames (e.g. image frames forming according to the first digital signal 9a and the second digital signal 9b) so that one processed image frame is obtainable (e.g. an image frame formed according to the third digital signal 9c).
Even though the subtraction between the digital signals mentioned above is not calculated, sensing data corresponding to all the sensing pixels are automatically read from the image sensor 91 to be stored in a digital buffer, which is generally a SRAM, after the exposure of the sensing pixels. And the stored sensing data is waited to be read by an external device.